The present invention relates to an arrangement for charging an accumulator. The arrangement is suitable for use in for example working machines.
Working machines, such as wheel loaders and excavators and other construction equipment are often equipped with a hydraulic oil pressure system that powers e.g. the boom operating system and the arm and bucket lifting system. The hydraulic system may be equipped with a suspension system in order to take up load variations and to relieve the system from sharp impulses. On wheel loaders, such a system is used to damp the boom and is referred to as a boom suspension system (BSS). The BSS system utilities a gas/oil accumulator connected to the lift cylinders to absorb shocks and smooth out rough roads. This in turn allows for faster cycle times and less spillage and will also increase the comfort of the operator. When the boom suspension system is activated, the accumulator is connected to the lift cylinder system.
The boom suspension system can however not be used constantly. During some of the operations performed by the vehicle, the BSS system must be deactivated in order to obtain a high precision and a high stiffness of the system, e.g. when loading gravel or handling pallets. The boom suspension system can thus be deactivated temporarily by the vehicle control system when certain operations are chosen by the operator. One common operation when the boom suspension system is deactivated is when the kick down of the vehicle is engaged. The kick down function is selected by engaging a switch which at the same time engages the first gear of the vehicle. The kick down is used when loading heavy material such as gravel, stones or the like, in order to obtain a high breakout torque.
When the boom suspension system is deactivated, the pressure in the accumulator and the lift cylinders will no longer correspond to each other. When the boom suspension system is deactivated, the different pressures are of no concern. However, it is important that the pressure is the same in both the accumulator and the lift cylinders when the boom suspension system is activated again. If the two systems have different pressures, the boom will move some in an uncontrolled way. If the pressure is higher in the accumulator, the boom would move upwards some and if the pressure in the accumulator is lower, the boom would drop.
Most often in actual operations, the pressure in the lift cylinders is relatively low or close to zero when the boom suspension system is deactivated by the vehicle control system. This is because the first gear, i.e. the kick down, is mostly used when loading and on those occasions, the bucket is lowered, which means that the pressure in the lift cylinders is low. After the loading, the bucket is full and is raised some, which means that the pressure in the lift cylinders is high. It would be detrimental to connect the high pressure lift cylinders to an empty accumulator or to an accumulator having a much lower pressure.
For this reason, the accumulator is commonly charged to a high pressure when the boom suspension system is deactivated. The accumulator is then drained during activation of the boom suspension system such that the pressure in the accumulator corresponds to the pressure in the lift cylinders prior to the connection of the accumulator to the lift cylinders. In a common system, the accumulator is charged to approximately 150 bars. Since the pressure in the lift cylinders is normally lower than 150 bars when the BSS is activated, it is ensured that the pressure in the accumulator will be sufficient for a smooth activation of the BSS. The charging of the accumulator is done by using the hydraulic main pump that powers the lift cylinders. A pressure regulator valve is used to limit the charging pressure of the accumulator to the predefined pressure, e.g. to 150 bars. A pressure balancing circuit is used to balance the pressure in the accumulator to the pressure of the lift cylinder when the BSS is activated.
One drawback with the current charging system is that energy is lost each time oil is drained from the accumulator to balance the pressure. This in turn increases the fuel consumption of the vehicle. The accumulator is often filled at maximum pump pressure which causes further pressure losses. There is thus a need for an improved accumulator charging arrangement.
It is desirable to provide an improved arrangement for charging an accumulator of a vehicle. It is also desirable to provide an improved method for charging an accumulator of a vehicle.
In an arrangement for charging an accumulator with hydraulic fluid, wherein the accumulator is used in a hydraulic boom suspension system for a lift arm of a vehicle, the arrangement comprising a first hydraulic pump arranged to supply hydraulic fluid to a hydraulic cylinder system of the lift arm, and a second hydraulic pump arranged to supply hydraulic fluid to a second hydraulic system, the second hydraulic pump is further arranged to supply hydraulic fluid for charging the accumulator.
By the accumulator charging arrangement according to the invention, an arrangement in which an accumulator is charged by a second hydraulic oil pump, which is arranged for driving a second hydraulic system, is used to charge the accumulator is obtained. In this way, the first hydraulic oil pump, which may be a main pump, is relieved from the accumulator charging which means that the full capacity of the first pump can be used for the handling of an implement, such as a bucket during loading. At the same time, it is easier to set and control the second hydraulic pump individually to the requirements for charging the accumulator.
In an advantageous development of the inventive accumulator charging arrangement, the second hydraulic pump is arranged to supply hydraulic fluid to a hydraulic motor for driving a cooling fan of the vehicle. Since the hydraulic pump for the cooling fan is designed to handle the cooling of the vehicle at extreme temperatures and loads, the cooling fan pump will have capacity to spare in most of the cases. By temporarily disengaging the cooling fan from the second hydraulic pump, the second hydraulic pump can be controlled specifically for the charging of the accumulator.
In an advantageous development of the inventive accumulator charging arrangement, the second hydraulic pump is arranged to be controlled by a pressure regulator valve, by which the pressure of the second hydraulic pump can be set by an electric signal from an ECU. In this way, the ECU can control the charging process in an energy efficient manner. By setting a required pressure level for the second hydraulic pump, excessive charging of the accumulator can be avoided. A further advantage is that the pressure loss is lower when the accumulator is charged with a lower pressure difference.
In an advantageous development of the inventive arrangement, the arrangement comprises a first charging function adapted to charge the accumulator to a first pressure when the boom suspension system is inactivated and a second charging function adapted to charge the accumulator to a second pressure when a control signal indicates that the boom suspension system is to be activated. The advantage of dividing the charging process of the accumulator is that a first charging step can be performed when the boom suspension system is in an inactive state. By pre-charging the accumulator to a first pressure, the accumulator will be partly charged when the boom suspension system is to be activated. The charging to the required pressure will thus take shorter time. When the boom suspension system is to be activated, a control signal is sent to the ECU. The ECU will complete the charging of the accumulator before the accumulator is connected to the lift cylinder system. One advantage of this is that the delay when activating the boom suspension system will be reduced. Another advantage is that the accumulator will not be overcharged which will preserve energy. A further advantage is that the accumulator is charged with a more moderate pressure difference. In conventional boom suspension systems, the accumulator is overcharged by the main hydraulic pump and the excessive pressure is drained before the accumulator is connected to the lift arm hydraulic cylinder system.
The first pressure is preferably set to a value that is most often below the pressure in the lift cylinder system when the boom suspension system is to be activated, i.e. the pressure in the lift cylinder system when the bucket is loaded. By setting the first pressure to a value below the required value, there is no need to drain hydraulic oil from the accumulator. For a typical wheel loader, the first pressure is preferably in the range between 80 and 120 bar. The second pressure is preferably substantially equal to the current load pressure in the lift arm hydraulic cylinder system when the boom suspension system is to be activated. In this way, there will be no need to drain the accumulator of excessive hydraulic oil. By connecting the accumulator to the lift arm hydraulic cylinder system when the pressure in the accumulator is substantially equal to the current load pressure in the lift arm hydraulic cylinder system, the activation of the boom suspension system will be smooth without sudden unexpected moves of the lift arms.
In an advantageous development of the inventive arrangement, the first and/or the second charging function comprises a predefined pressure ramp function. In this way, the charging of the accumulator can be performed in an energy efficient way with reduced losses. By adapting the pressure ramp function to the rotational speed of the second hydraulic pump, the losses can be reduced further. It is also possible to adapt the pressure ramp function to other parameters, such as the hydraulic oil temperature, in order to optimize the charging process further.
In an advantageous development of the inventive arrangement, the arrangement further comprises a valve means that is adapted to connect the first hydraulic pump to the accumulator and to disconnect the second pump from the accumulator when a temperature signal for the cooling fan is above a predefined level and/or when a brake pressure signal is below a predefined level. In this way, the accumulator can be charged even in extreme situations, e.g. when the cooling system is at its maximum capacity. Instead of delaying the charging of the accumulator, the first pump is used to charge the accumulator in a conventional manner, with an overcharging of the accumulator and with a subsequent draining of the pressure to the appropriate pressure level.
In an advantageous development of the inventive arrangement, the valve means is spring-loaded such that the first hydraulic pump is connected to the accumulator and that the second pump is disconnected from the accumulator when the valve means receives no electrical signal. In this way, the accumulator can be charged even when the cabling to the valve means is broken or when the ECU output to the valve means is broken.
In an advantageous method for charging an accumulator with hydraulic fluid, where the accumulator is used in a hydraulic boom suspension system for a lift arm of a vehicle, and where the lift arm is provided with a hydraulic cylinder system, the steps of charging the accumulator to a first pressure when the boom suspension system is inactivated, and charging the accumulator to a second pressure when a control signal indicates that the boom suspension system is to be activated are comprised. By dividing the charging of the accumulator into two steps, an energy preserving accumulator charging method is obtained, which at the same time minimizes the delay when activating the boom suspension system. The control signal may e.g. be generated when a reverse gear of the vehicle is activated.
In an advantageous development of the inventive method, the second pressure is substantially equal to the current load pressure in the lift arm hydraulic cylinder system. By limiting the second pressure to the actual pressure in the lift arm hydraulic cylinder system, excessive charging is avoided. By connecting the accumulator to the lift arm hydraulic cylinder system when the second pressure is substantially equal to the current load pressure in the lift arm hydraulic cylinder system, a smooth activation of the boom suspension system is obtained, with no unexpected moves of the lift arms.
In an advantageous development of the inventive method, the accumulator is charged according to a ramp function. In this way, the charging of the accumulator can be adapted to the hydraulic system of the vehicle.
In an advantageous development of the inventive method, a first hydraulic pump is arranged to supply hydraulic fluid to the lift arm hydraulic cylinder system, and a second hydraulic pump is arranged to supply hydraulic fluid to a hydraulic motor for driving a cooling fan of the vehicle, and to charge the accumulator to the first pressure and the second pressure. By using the hydraulic pump that is used to power the cooling fan, the first pump can be used solely for the hydraulic lift system.
In an advantageous development of the inventive method, the second hydraulic pump is controlled by a pressure regulator valve, whereby the pressure of the second hydraulic pump is set by an electric signal. In this way, the charging of the accumulator can be optimized by reducing the losses in the pump.
In an advantageous development of the inventive method, the flow of the second hydraulic pump is controlled by controlling the displacement of the second hydraulic pump, whereby the flow of the second hydraulic pump is set by an electric displacement signal. In this way, the charging of the accumulator can be optimized by reducing the losses in the pump.
By charging the accumulator according to a ramp function, the losses can be reduced further. The ramp function can be adapted to operating parameters of the second hydraulic pump, such as the rotational speed of the second hydraulic pump. The temperature of the hydraulic oil and/or the outer temperature can also be used as an input parameter.
In an advantageous development of the inventive method, the first hydraulic pump is connected to the accumulator and the second pump is disconnected from the accumulator when a temperature signal for the cooling fan is above a predefined level and/or when a brake pressure signal is below a predefined level. In this way, the accumulator can be charged also when the second hydraulic pump is fully occupied. The accumulator can thus be charged without having to wait for the second hydraulic pump to be available. This reduces the delay time for the activation of the boom suspension system.